[JRTJH]

I "mis-spoke" in post #14. What I said was:

The blue wire in the TRUCK umbilical is the "brake feed" for the trailer electric brakes. It connects to one of the black wires. The other two black wires "should be the emergency breakaway switch wires"... One will be connected to the battery feed and the other will be connected to the ground terminal.

What I should have said is: One will be connected to the battery feed and the other will be connected to the trailer brake wire, which also connects to the blue wire from the truck umbilical. It is not connected to ground. The ground is provided "through the trailer brake magnets".

Yes, the break-away switch is mounted to the side if the trailer A-frame. There are two black wires coming from it (probably two of the black wires you're trying to identify)... There is a lanyard attached to a plastic pin that's inserted into the break-away switch. If you pull the lanyard, the pin pulls out of the switch. This causes the spring loaded contacts in the switch to close, completing the circuit from the battery to the brake wire, activating the trailer brakes in the event of a tow vehicle/trailer separation.

Once you pull the lanyard and the pin comes out of the break-away switch, be sure to reinsert the pin to "deactivate" the switch, interrupting power to the brakes or you'll probably melt the break-away switch and/or burn up the wiring in your brake magnets inside the hubs. That switch is an "emergency, last resort safety device" and it is not designed to apply "long term power" to the brakes. It's a "one time, last resort, probably will be destroyed while trying to stop a run-away trailer" kind of device....